MOT emissions failure (Mondeo)

My lad's 1999 2.0 Mondeo has just failed its MOT with:-

Exhaust emissions carbon monoxide content at idle (0.507... limit 0.300)

Exhaust emissions carbon monoxide content after 2nd fast idle (0.523... limit 0.300)

The cat was replaced about a year ago. Are we looking at a knackered lambda or could it be something else?

Without any more info then that's the best guess. Presumably it's not burning bugger loads of oil?

If you know the lambda reading, then this diagnosis aid I found a while ago might help.......

Basic fault finding for advanced emission test failures:

If a vehicle fails for high CO at idle and /or fast idle then.... if Lambda is too low, the mixture is too rich. This can be caused either by a misfire or by a faulty Lambda sensor. If it's running very rich, the 02 and HC will be higher than normal.
if it has a high Lambda and high 02 (0.5 to 1.5%), the "CAT" could be faulty. Before condemning it make sure its hot enough to work, the Lambda and other sensors are working OK and the exhaust downpipe hasn't got any leaks.
if the vehicle fails for high HC then ...if the CO passes, the vehicle is probably misfiring. Or if the CO fails and the Lambda reading is too low, the mixture is probably too rich and it may also be misfiring.
if the CO fails, the Lambda reading is high and the O2 reading is high (0.5 to 1.5%) the "CAT" could be faulty. Again check out the simple measures above before condemning it.
if the Lambda fails then...if its too high the Lambda sensor may be faulty, the mixture may be too weak or there may be a hole in the exhaust downpipe. If Lambda is too low the sensor may be faulty, the mixture may be rich or the vehicle may be misfiring.

In message , Steve writes

[snip]

Thanks for that Steve and also to Duncan.

Out of interest if it was burning lots of oil Duncan what would that point to ?

*curiosity hat on today* a few hours on how stuff works has gotten the brain ticking over.

was it actually fully hot when tested?

Knackered bores or piston rings, most likely.

Or valve stem seals/valve guides.

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